Theory and Simulation
Conditions in the vast universe can be quite extreme: Violent collisions scar the surfaces of planets. Nuclear reactions in bright stars generate tremendous amounts of energy. Gigantic explosions catapult matter far out into space. But how exactly do processes like these unfold? What do they tell us about the universe? And could their power be harnessed for the benefit of humankind?
To find out, researchers from SLAC's High Energy Density Science (HEDS) division perform sophisticated experiments and computer simulations that recreate violent cosmic conditions on a small scale in the lab. Three recent studies exemplify this approach, shining light on meteor impacts, the cores of giant planets and cosmic particle accelerators a million times more powerful than the Large Hadron Collider, the largest particle racetrack on Earth.
High pressure can turn a soft form of carbon – graphite, used as pencil lead – into an extremely hard form of carbon, diamond. Could the same thing happen when a meteor hits graphite in the ground? Scientists have predicted that it could, and that these impacts, in fact, might be powerful enough to produce a form of diamond, called lonsdaleite, that is even harder than regular diamond. The team heated the surface of graphite with a powerful optical laser pulse that set off a shock wave inside the sample and rapidly compressed it. By shining bright, ultrafast X-rays from SLAC’s X-ray laser Linac Coherent Light Source (LCLS) through the sample, the researchers were able to see how the shock changed the graphite’s atomic structure.
A second study, published in Nature Communications, looked at another peculiar transformation that might occur inside giant gas planets like Jupiter, whose interior is largely made of liquid hydrogen: At high pressure and temperature, this material is believed to switch from its “normal,” electrically insulating state into a metallic, conducting one. Current Director of SLAC's HEDS division Siegfried Glenzer and his fellow scientists performed an experiment at Lawrence Livermore National Laboratory (LLNL), where they used the high-power Janus laser to rapidly compress and heat a sample of liquid deuterium, a heavy form of hydrogen, and to create a burst of X-rays that probed subsequent structural changes in the sample. The team saw that above a pressure of 250,000 atmospheres and a temperature of 7,000 degrees Fahrenheit, deuterium indeed changed from a neutral, insulating fluid to an ionized, metallic one.
In addition to planetary science, the study could also inform energy research aimed at using deuterium as nuclear fuel for fusion reactions that replicate analogous processes inside the sun and other stars.